BEIJING — Authorities are confident that a series of underwater signals in the Indian Ocean are coming from a missing Malaysia Airlines plane, Australia’s prime minister said Friday, while warning that those signals are beginning to fade.

Tony Abbott told reporters in the Chinese city of Shanghai that searchers have narrowed the area where they hoped to locate the wreckage of the plane on the ocean floor by listening for pings from emergency locator beacons built into the plane’s flight data and cockpit voice recorders.

Audio transmissions were picked up by the Australian navy vessel Ocean Shield in four separate locations on April 5 and April 8, using a Towed Pinger Locator supplied by the US Navy and dangled deep in the sea.